This invention relates to cardiac defibrillators and more particularly to defibrillators powered from alternate power sources.
Cardiac defibrillators generate a high voltage output that is discharged from a capacitor into a patient to perform either defibrillation or synchronized cardioversion. The life of a patient may depend upon a defibrillator's reliability and whether it can generate sufficient energy in a limited amount of time. A defibrillator charger circuit, which operates from a single power supply, is described in Hewlett-Packard operating manual, 43100A Defibrillator/Monitor and Recorder, May 1988, Part No. 43100-91909 and is incorporated by reference.
It is desirable that the defibrillator have the capacity to operate in different situations; for example, in a hospital emergency room or in remote locations such as an ambulance. It is therefore necessary that the defibrillator's high voltage charging transformer be able to efficiency receive and transform power from DC power supplies with different output characteristics. For example, the transformer should typically be capable of receiving power from either an AC-to-DC converter (e.g. for 120 volt AC "plug in" type source) or from a DC battery.
Several problems are associated with designing a defibrillator to operate from more than one power supply. For example, power supply switching circuitry operating at high current levels reduces the reliability of the overall defibrillator system. Fault conditions on one of the power supplies may alter the normal output characteristics of the second power supply. Due to the high current from the power supply to the transformer, it is also necessary that the switching circuit have low energy loss characteristics. However, components that have high current ratings and provide low energy loss are expensive and typically require additional logic circuitry. In addition, an AC-to-DC converter is typically not capable of supplying as much peak current as a battery, so a transformer that is optimized for the battery is not optimized for the AC-to-DC converter. This difference can adversely affect the amount of time required to properly charge the defibrillator's storage capacitor.
Accordingly, a need exists for a cardiac defibrillator that is reliable, capable of producing a high voltage output in a minimal amount of time, and inexpensive while capable of being energized from multiple power supplies.